1. Field of the Invention
This invention relates to an improvement of a back-scattered electron detector for detecting back-scattered electrons, employed in an electron microscope or an electron beam exposure system which is used in manufacturing semiconductor devices such as large-scale integrated circuit devices.
2. Description of the Prior Art
For a back-scattered electron detector, several ideas have been proposed and tried, one of which is a semiconductor type detector having a PN junction from which electric signals may be generated in accordance with the amount of back-scattered electrons colliding with the PN junction. Such a type detection has been disclosed in the paper titled "Computer Controlled Electron Microfabrication Machine With A New Registration System" of Journal of Physics E: Scientific Instruments 1974, Volume 7 (1974), and also is available as a semiconductor type detector, "Si Surface Barrier Detector", a product of Oltec Company, U.S.A.
Another type of a back-scattered electron detector has been proposed by the inventors of the present invention. The porposed detector comprises phosphor which emits light outputs by being excited by electrons, a light guide and a photo-multiplier, and as a light guide a glass prism is used to guide the emitted light outputs to the photo-multiplier.
Meanwhile, in view of the function of an electron microscope or an electron beam exposure system, high speed detecting of electrons is required in a back-scattered electron detector. For this requirement, a semiconductor type detector such as explained above is not adequate. The detecting speed of the semiconductor type detector is not satisfactorily high enough as required, due to the delay based on the inherent capacitance of the device, itself, while the detector proposed by the inventors of the present invention has been improved in high-speed detection of electron beams.
In an electron microscope or an electron beam exposure system, focusing lenses and deflection coils are provided respectively to focus and to deflect electron beams. A final focusing lens is important as it is used for finally focusing electron beams on the surface of a specimen to be observed in an electron microscope or to be exposed with electrons in an electron beam exposure system.
There are two types of arrangements of a deflection coil and the final focusing lens, one of which being that the deflection coil is before or in the final focusing lens so that an ample space between the final lens and the specimen is available, which space is used for a back-scattered electron detector. The other type of arrangement is called a post-lens deflection type, and of some advantage in designing the focusing and deflection of electron beams, compared with the former type. In the post-lens deflection type, the deflection coil is after the final focusing lens, that is, between the final focusing lens and the specimen. Therefore, as can be easily understood, the post-lens type has difficulty in receiving a back-scattered electron detector between the deflection coil and the specimen as the space therebetween is scanty. In this regard, the above mentioned detector proposed by the inventors of the present invention also is not suitable for use in the post-lens deflection type because the thickness of a glass prism is comparatively too large for the space between the deflection coil and the specimen.